1. Field of the Invention
The present invention relates to photographic cameras and more particularly to apparatus for moving a lens in such a camera.
2. Discussion Related to the Problem
Various schemes have been proposed for automatically determining the required focus adjustment in a photographic camera. A feature common to these schemes is a means for moving a lens element in the camera during or after focus determination. Other cameras include "zoom" lenses, wherein the magnification of the camera lens is varied by moving an element in the lens. Usually movement of the lens element has been accomplished by means of a servo including a rotating electric motor. For example, see U.S. Pat. No. 3,971,395 issued Nov. 4, 1975 to F. T. Ogawa. A servo system using a rotating electric motor to move a lens element in a photographic camera is sub-optimum from the standpoint of cost, size, speed of operation, complexity and power consumption. It is known to use motors other than rotating electric motors in lens moving servo systems for other kinds of optical apparatus. U.S. Pat. No. 3,997,715 issued Dec. 14, 1976 to James E. Elliott discloses the use of a linear motor comprising a speaker coil type drive for use in the focus system of a video disc player. A problem associated with the use of a speaker coil type linear motor in a photographic camera is the relatively high standby power consumption required to hold the lens in the proper focus position during exposure. The '715 patent also suggests the use of "piezoelectric motive means", although further details are not given. It is known that piezoelectric linear motors are relatively small, rugged, simple, fast and when moved to a particular position, draw a minimum of standby power. Unfortunately, there are also problems associated with the use of piezoelectric motors to move lenses in photographic cameras. For one thing, the amount of motion obtainable from a piece of piezoelectric material is relatively small. It is not nearly enough, without some form of mechanical amplification, to move a camera lens a reasonable focusing distance with a reasonably sized piezoelectric element. The other main drawback, which appears to be even more serious, is the fact that piezoelectric materials require relatively high fields to operate; power supplies of 200 to 300 volts being the norm. The added cost of including a 300 volt power supply in a photographic camera just to power a piezoelectric motor would probably outweigh any cost advantages that could be achieved by replacing a rotating electric motor with a piezoelectric motor. These problems, among others, are solved by the present invention.